1. Field of the Invention
The present invention relates to a toner for use in developing an electrostatic latent image. In addition, the present invention also relates to a method for developing an electrostatic latent image, a method for fixing a toner image, an image forming method using the toner and a process cartridge using the toner.
2. Discussion of the Background
Among the methods for fixing toner images upon application of heat thereto, methods using a heat roller are widely used because of having good energy efficiency. Recently, the heat energy used for fixing toner images becomes smaller and smaller to fix toner images at a low temperature, i.e., to save energy. In addition, it is needed to decrease the warm-up time of an image forming apparatus, while the electric power consumption of the apparatus is minimized in the waiting state to protect environment. Requirements for next generation image forming apparatuses are described in the DSM (Demand-side Management) program of IEA (International Energy Agency). There are several requirements therein such that the warm-up time should not be greater than 10 seconds and the power consumption in a waiting state should be not greater than 10 to 30 watt (which changes depending on the copying speed) in copiers having a copy speed not less than 30 cpm (copies per minutes). In order to fulfill these requirements, the power consumption of copiers must be dramatically reduced.
In attempting to fulfill the requirements, fixing devices using a fixing element, such as heat rollers, which has a low thermal capacity and which has a quick temperature rising property, have been studied. However, the devices do not bring about desirable effects.
Therefore, it is essential for fulfilling the requirements to develop a toner which can be fixed at a low temperature, i.e., which can be used in an image forming apparatus performing fixing at a relatively low temperature.
In attempting to develop a toner having a low temperature fixability, techniques such that a specific non-olefin type crystalline polymer or a specific crystalline polyester resin, which has a sharp melting property at its glass transition temperature (Tg) such that the resin not only changes its crystal form at Tg but also changes its state from a solid state to a liquid state (i.e., decreases its viscosity), is included in the binder resin have been disclosed.
However, when such a resin having a sharp melting property is kneaded together with other toner constituents, such as colorants and release agents, to prepare a toner, the melt viscosity of the kneaded mixture seriously decreases, thereby causing a problem in that the colorants and release agents cannot be finely dispersed. When colorants are not well dispersed, problems occur in that high density images cannot be produced and clear images cannot be produced in color image production. In this case, when the colorants are materials having a low resistivity, such as carbon black, the resultant toner also has a low resistivity, thereby causing problems in that the resultant toner images have background fouling (due to deterioration of the developing property of the toner) and uneven density in solid images (due to deterioration of the transfer property of the toner).
When the release agents are not well dispersed, the release agents tend to present on the surface of the resultant toner, thereby deteriorating the developing property of the toner, resulting in occurrence of the background fouling problem. In addition, such toners contaminate the carriers which are used together with the toners to prepare two-component developers and/or charging rollers or blades, and thereby the life of such image forming members is shortened.
With respect to the techniques in which a crystalline resin is included in the binder resin, crystalline resins hardly disperse colorants, and thereby the above-mentioned problems tend to occur. In addition, crystalline resins are typically insoluble in resins which are generally used as the toner binder resin. Therefore, even when a crystalline resin is used in combination with a resin which is used as the toner binder, problems in that the colorant used together with the crystalline resin and the binder resin cannot be well dispersed in the resins because the colorant such as carbon blacks and pigments is not included in the crystalline resin (i.e., the colorant is selectively included in the binder resin).
Recently, the particle diameter of toner becomes smaller and smaller to produce high quality toner images. When toner constituents are not well dispersed in such a toner having a small particle diameter, the above-mentioned problems caused by the uneven dispersion becomes more serious.
In addition, when a polyester having crystallinity is included in the polyester resin in an amount greater than a certain amount, problems which occur are that the resultant toner has poor offset resistance and preservability; and when the toner is manufactured by a pulverization method, the kneaded mixture cannot be finely pulverized. Similarly to the case where a wax serving as a release agent establishes a trade off relationship between releasability and colorant dispersing ability (i.e., a trade off relationship in that when the content of a wax is increased, the releasability of the resultant toner can be improved but the colorant dispersing ability of the toner deteriorates), the polyester resins having crystallinity establish a trade off relationship between low temperature fixability and colorant dispersing ability even when the content thereof is changed.
The present inventors have disclosed a toner which includes a crystalline polyester and which has good sharp melting property (published unexamined Japanese Patent Application No. 2003-167384). However, a need exists for a toner having a further improved low temperature fixability.
In attempting to solve the uneven dispersion problem of a crystalline resin in toner, a toner which is prepared by agglomerating a particulate resin having crystallinity and a particle size on the order of submicrons together with fine particles of other toner constituents such as non-crystalline resins and release agents is disclosed in published unexamined Japanese Patent Application No. 2002-108018. However, in order to develop the effect of such a particulate resin in the resultant toner, the particle diameter of the resin dispersed in the toner should be not less than a certain particle diameter (i.e., when the particle diameter of the resin dispersed in the toner is too small, the effect of the resin cannot be well produced). Namely, there is a trade off relationship between the particle diameter of the resin dispersed in the toner and the dispersibility thereof. Namely, there is a limit to the low temperature fixability of such a toner.
In other words, it is essential to use a technique other than the technique using a crystalline resin, to impart good low temperature fixability to a toner.
Carbon blacks have been typically used as a colorant of black toners. However, in the case of magnetic toners, the magnetic material included therein often serves as a colorant (i.e., other colorants such as carbon black are not included therein). Such a magnetic toner often has a good low temperature fixability. The reason is considered to be that the magnetic material (typically, metal-containing materials) has relatively good heat conductivity compared to carbon blacks.
Although a toner including a magnetic material can be used as a one-component developer without causing problems, such a toner has poor developability when used for a non-magnetic toner (i.e., for a two-component developer) because the attraction force between the carrier used and the toner increases. When the addition amount of the magnetic material is decreased to reduce the attraction force, the resultant toner has an insufficient low temperature fixability.
In addition, a need exists for a toner including no carbon black in view of safety.
Because of these reasons, a need exists for a toner which is safe and has such an excellent low temperature fixability as to be used for low heat capacity fixing devices without causing an offset problem and a preservation problem and which can produce good images without causing background fouling problem and a toner scattering problem in that toner particles are scattered in image forming apparatus, resulting in contamination of image forming members.